6,284 research outputs found
Two-body effects in the decay rate of atomic levels
Recoil corrections to the atomic decay rate are considered in the order of
Zm/M . The expressions are treated exactly without any expansion over Z alpha.
The expressions obtained are valid both for muonic atoms (for which they
contribute on the level of a few percent in high Z ions) and for electronic
atoms. Explicit results for Lyman-alpha transitions for low-Z of the order
(Zm/M)(Z alpha)^2 are also presented.Comment: 5 pages, 1 table, email: [email protected]
Determination of complex dielectric functions of ion implanted and implantedâannealed amorphous silicon by spectroscopic ellipsometry
Measuring with a spectroscopic ellipsometer (SE) in the 1.8â4.5 eV photon energy region we determined the complex dielectric function (Ï” = Ï”1 + iÏ”2) of different kinds of amorphous silicon prepared by selfâimplantation and thermal relaxation (500â°C, 3 h). These measurements show that the complex dielectric function (and thus the complex refractive index) of implanted aâSi (iâaâSi) differs from that of relaxed (annealed) aâSi (râaâSi). Moreover, its Ï” differs from the Ï” of evaporated aâSi (eâaâSi) found in the handbooks as Ï” for aâSi. If we use this Ï” to evaluate SE measurements of ion implanted silicon then the fit is very poor. We deduced the optical band gap of these materials using the DavisâMott plot based on the relation: (Ï”2E2)1/3 ⌠(Eâ Eg). The results are: 0.85 eV (iâaâSi), 1.12 eV (eâaâSi), 1.30 eV (râaâSi). We attribute the optical change to annihilation of point defects
Stellar Dynamics and the implications on the merger evolution in NGC6240
We report near-infrared integral field spectroscopy of the luminous merging
galaxy NGC 6240. Stellar velocities show that the two K-band peaks separated by
1.6arcsec are the central parts of inclined, rotating disk galaxies with equal
mass bulges. The dynamical masses of the nuclei are much larger than the
stellar mass derived from the K-band light, implying that the progenitor
galaxies were galaxies with massive bulges. The K-band light is dominated by
red supergiants formed in the two nuclei in starbursts, triggered ~2x10^7 years
ago, possibly by the most recent perigalactic approach. Strong feedback effects
of a superwind and supernovae are responsible for a short duration burst
(~5x10^6 years) which is already decaying. The two galaxies form a
prograde-retrograde rotating system and from the stellar velocity field it
seems that one of the two interacting galaxies is subject to a prograde
encounter. Between the stellar nuclei is a prominent peak of molecular gas
(H_2, CO). The stellar velocity dispersion peaks there indicating that the gas
has formed a local, self-gravitating concentration decoupled from the stellar
gravitational potential. NGC 6240 has previously been reported to fit the
paradigm of an elliptical galaxy formed through the merger of two galaxies.
This was based on the near-infrared light distribution which follows a
r^1/4-law. Our data cast strong doubt on this conclusion: the system is by far
not relaxed, rotation plays an important role, as does self-gravitating gas,
and the near-infrared light is dominated by young stars.Comment: 34 pages, 11 figures, using AASTEX 5.0rc3.1, paper submitted to the
Astrophysical Journal, revised versio
Probability distributions for quantum stress tensors in four dimensions
We treat the probability distributions for quadratic quantum fields, averaged
with a Lorentzian test function, in four-dimensional Minkowski vacuum. These
distributions share some properties with previous results in two-dimensional
spacetime. Specifically, there is a lower bound at a finite negative value, but
no upper bound. Thus arbitrarily large positive energy density fluctuations are
possible. We are not able to give closed form expressions for the probability
distribution, but rather use calculations of a finite number of moments to
estimate the lower bounds, the asymptotic forms for large positive argument,
and possible fits to the intermediate region. The first 65 moments are used for
these purposes. All of our results are subject to the caveat that these
distributions are not uniquely determined by the moments. However, we also give
bounds on the cumulative distribution function that are valid for any
distribution fitting these moments.We apply the asymptotic form of the
electromagnetic energy density distribution to estimate the nucleation rates of
black holes and of Boltzmann brains.Comment: 26 pages, 2 figure
Stimulated Raman spin coherence and spin-flip induced hole burning in charged GaAs quantum dots
High-resolution spectral hole burning (SHB) in coherent nondegenerate
differential transmission spectroscopy discloses spin-trion dynamics in an
ensemble of negatively charged quantum dots. In the Voigt geometry, stimulated
Raman spin coherence gives rise to Stokes and anti-Stokes sidebands on top of
the trion spectral hole. The prominent feature of an extremely narrow spike at
zero detuning arises from spin population pulsation dynamics. These SHB
features confirm coherent electron spin dynamics in charged dots, and the
linewidths reveal spin spectral diffusion processes.Comment: 5 pages, 5 figure
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Exp6-polar thermodynamics of dense supercritical water
We introduce a simple polar fluid model for the thermodynamics of dense supercritical water based on a Buckingham (exp-6) core and point dipole representation of the water molecule. The proposed exp6-polar thermodynamics, based on ideas originally applied to dipolar hard spheres, performs very well when tested against molecular dynamics simulations. Comparisons of the model predictions with experimental data available for supercritical water yield excellent agreement for the shock Hugoniot, isotherms and sound speeds, and are also quite good for the self-diffusion constant and relative dielectric constant. We expect the present approach to be also useful for other small polar molecules and their mixtures
On the thermodynamics of the SwiftâHohenberg theory
We present the microbalance including the microforces, the first- and second-order microstresses for the SwiftâHohenberg equation concomitantly with their constitutive equations, which are consistent with the free-energy imbalance. We provide an explicit form for the microstress structure for a free-energy functional endowed with second-order spatial derivatives. Additionally, we generalize the SwiftâHohenberg theory via a proper constitutive process. Finally, we present one highly resolved three-dimensional numerical simulation to demonstrate the particular form of the resulting microstresses and their interactions in the evolution of the SwiftâHohenberg equation
Monte Carlo Simulations for Ghost Imaging Based on Scattered Photons
X-ray based imaging modalities are widely used in research, industry, and in
the medical field. Consequently, there is a strong motivation to improve their
performances with respect to resolution, dose, and contrast. Ghost imaging (GI)
is an imaging technique in which the images are reconstructed from measurements
with a single-pixel detector using correlation between the detected intensities
and the intensity structures of the input beam. The method that has been
recently extended to X-rays provides intriguing possibilities to overcome
several fundamental challenges of X-ray imaging. However, understanding the
potential of the method and designing X-ray GI systems pose challenges since in
addition to geometric optic effects, radiation-matter interactions must be
considered. Such considerations are fundamentally more complex than those at
longer wavelengths as relativistic effects such as Compton scattering become
significant. In this work we present a new method for designing and
implementing GI systems using the particle transport code FLUKA, that rely on
Monte Carlo (MC) sampling. This new approach enables comprehensive
consideration of the radiation-matter interactions, facilitating successful
planning of complex GI systems. As an example of an advanced imaging system, we
simulate a high-resolution scattered photons GI technique
Off the Beaten Path: Let's Replace Term-Based Retrieval with k-NN Search
Retrieval pipelines commonly rely on a term-based search to obtain candidate
records, which are subsequently re-ranked. Some candidates are missed by this
approach, e.g., due to a vocabulary mismatch. We address this issue by
replacing the term-based search with a generic k-NN retrieval algorithm, where
a similarity function can take into account subtle term associations. While an
exact brute-force k-NN search using this similarity function is slow, we
demonstrate that an approximate algorithm can be nearly two orders of magnitude
faster at the expense of only a small loss in accuracy. A retrieval pipeline
using an approximate k-NN search can be more effective and efficient than the
term-based pipeline. This opens up new possibilities for designing effective
retrieval pipelines. Our software (including data-generating code) and
derivative data based on the Stack Overflow collection is available online
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